Homebrew Python 3.5 transition

August 31, 2015

Python 3.5 is scheduled to be released on September 13, 2015. Since Homebrew carries the newest stable version of its packages, the Homebrew python3 formula will adopt Python 3.5 very quickly upon its release. This post explains how migrating to Python 3.5 will affect Homebrew users who have set up a Python 3 development environment using Homebrew packages.

Packages installed with pip will not be found and scripts will break

Python packages which were installed against Python 3.4 with pip or easy_install will need to be reinstalled for Python 3.5. Packages installed against Python 3.4 will not be visible from Python 3.5 because Python uses versioned site-packages directories. Scripts (i.e. ipython3) which were installed with pip or easy_install using Python 3.4 will stop working until they are reinstalled.

To reconstitute your Python environment, consider running pip3 freeze > site-requirements.txt before you upgrade to save a list of your installed packages. You should edit this list by hand to remove any packages which are managed by Homebrew. Then, you can run pip3 install -r site-requirements.txt after you upgrade to reinstall your environment.

(If you forget to do this before you upgrade, you can run $(brew --cellar)/python3/3.4.3_2/bin/python3 -m pip freeze to generate the list of packages 3.4 knows about, even after 3.5 is installed.)

Homebrew packages built with --with-python3 will break

Homebrew will take care of rebuilding the relatively few Homebrew packages which have mandatory or recommended dependencies on "python3" or :python3. If you are only using these packages, you won’t need to do anything. (The complete list of those packages is: circlator, gubbins, iva, keepassc, lensfun, libsigrokdecode, mypy, pastebinit, ponysay, pulseview, py3cairo, pymummer, pyqt5, retext, and xonsh.)

Packages which may be optionally built against python3 by passing --with-python3 will break and will need to be reinstalled by you. You can get a list of packages which were installed using --with-python3 using brew and jq like:

Running brew reinstall for each of those packages will bring your system up to date after the transition. (You can equivalently pipe the output into xargs brew reinstall.)

Continuing to use Python 3.4

Life on the bleeding edge isn’t for everyone. If you would prefer to continue using Python 3.4, please consider uninstalling the python3 formula and using pyenv instead. You can install pyenv with Homebrew with brew install pyenv. Pyenv is a great tool for managing multiple parallel Python installations.

I prefer not to accept Python versions into homebrew-versions because of the relatively high maintenance load, but starting a tap (Homebrew’s term for an alternative formula repository) is easy to do if you’d like to maintain a python34 formula yourself (or a suite of similar formulas, like Felix Krull’s deadsnakes PPA for Ubuntu). I would owe you a beer. :)

Migrating early

If you would like to transition to Python 3.5 before its official release, you can run brew install python3 --devel to install the release candidate. After 3.5 final is released, a normal brew update && brew upgrade will upgrade you to the release version. Anything that works with a pre-release version of 3.5 should continue working with a release version of 3.5.

Questions

If you need help, please feel free to reach out on Homebrew’s Github issue tracker or the mailing list (homebrew@librelist.com). Thanks for your patience!